Trad. Rudder Construction/Redesign

[Bogle]

I have the original rudder, disassembled. I have the drawing of the constellation type (angular, with more area below and less area above (aloft?).) Thanks, Bill, for posting that.

I have compared the two. I would like to reduce weather helm, therefore I am planning on adding some area to the rudder to move the center of the underbody aft (what's the term here?).

I'm planning on a shape something between the two designs. Generally keeping the upper half's shape, but adding approximately 29 square inches to the original's area, below the center drift pin. I'm rounding off the constellation's sharp lower point so as not to create too much additional area. Also, I don't want to deviate from the original construction methods. The constellation type has a much longer moment arm, and would require either longer bolts or more drift pins. I'm already increasing the width of the #2, center, plank from 5" to 6". I'm reusing all the big bolts and the shafts -new nuts, washers, 5/16" drift pins and, of course, hand-picked (quartersawn) Honduran mahogany.

See the photo of my full size pattern at:
http://www.bway.net/~bogle/boat/newrud1/newrud1.html

The dark, bold line is my plan. The white line is the constellation shape and the faint curve in pencil is the original rudder trailing edge.

Comments, suggestions and knowledge are welcome. #92's original page is still at:
http://www.bway.net/~bogle/commander.html

Regards,
David
Brooklyn, New York
Pearson Commander #92, La Saladita

[ebb]

Capt. David,
well, I'll attempt a comment, if no mind.
Good luck on your great big round rudder!

One thing I've noticed in man's pursuit of speed is that modern rudders like to feather their trailing edges and avoid rounding things off, both trailing edges and especially the lower corner. In our boats, contrare to the Master, keeping it sharp would carry the keel aft to infinity (which I think was the real inspiration of the constellation rudder hung on the keel.)

If you keep to the original specs but attempt to feather the trailing edge the constellate rudder gets VERY thin down at the bottom. It's a long 27" skinny wedge starting at the shaft at 1" and ending at a point. I would guess the actual wood construction is closer to 1 3/8s - ending at 1/4 or fatter, don't know. That's my reading of the Alberg palimpsest, discounting the roundoff. THAT is a huge keel extension! And very difficult to stack using the bolts thru the shaft method of fastening. And, like you say, maybe impossible to keep from bending!

Perhaps the only problem with a larger rudder is actually stalling the boat, according to one commentator who bases his observation on the 'extreme' angle of the Alberg keel. One could argue (armed with appropriate brews) that the triangular rudder straightens the angle of the keel quite significantly.

I've convinced self that any rudder should get skinny at the trailing edge. But just how is big problem - I can't find anything on whether the blade had any shape. Did the half-a-heart blade diminish at all at the edge? Are all rudders off the keel merely flat. OR is there advantage to creating a mild foil? Keeping it 'fat' for a distance from the shaft would make it easier to keep wood over the bolts. It's certainly easier to see fairing the constellation. But rounded shapes have me stumped. Maybe that's why they're flat. ??

[Bill]

Take a look at this Web page:

http://www.well.com/user/pk/PCrudder.html

May have something useful . . .

[ebb]

That Emergency Rudder stuff is goode stuff indeed, requiring a new thread - if anybody interested here. One of these days gonna find me a gone-cruising site where emerging ruddors can be tossed around.

And for that matter: to find out of these keel-hung-rudder yot designers had any foil (read, lift) to their rudder designs. Have seen a more chatty version of what you posted, Bill, with pictures and all. And you can find number extensions of foil shapes on the net - but, I believe, all are for stand alone rudders and centerboards, not our tail-waggers

We've all seen that airplane wing illustration where the wind going over the hump on top creates lift. Right? OK, would someone please tell me what is gained when a shape is drawn thru the water and equal lift is created on both sides of symmetrical foil? Now, with the boat heeled over and exceeding her hull speed, you could say the humped side in the water was creating lift while the leaner exposed side had less resistence. Perhaps the symmetrical rudder being pulled by the helmsman is also creating more pressure on one side than the other and adding to the speed. Probably detracting. Yeah, so what is the least resistant streamlined shape of a keel hung rudder in moving water? I'm leaning toward that triangular rudder with that extra length extending the run of the keel like a sea fish.

Or, if you've ever squatted and sighted along the underbody from the forefoot to the rudder - you know, where the turn of the bilge and the gentle swelling of the smooth run of keel - THAT produces a sweet rush no science can touch. Which blade?

Whatever blows up yer trousers.

[Bill]

Maybe this will give you something to chew on:

http://dmawww.epfl.ch/Quarteroni-Cha...ericascup.php3

"Flow simulations have been employed for a number of years as a design optimization tool for high-performance racing yachts. Indeed, the EPFL has previously employed such simulations for the Merit Cup (Whitbread 1993) and FAST2000 (America's Cup 1999, see [1]) challenges. The mathematical equations describing three-dimensional incompressible turbulent flow are a complex set of coupled partial differential equations, whose resolution requires both accurate and robust numerical methods and substantial computational resources."

Not sure about those differential equations . . .

[ebb]

those guys, Do they like the water? Upright sleds with semi-detatched keels and dozens of serious well-heeled and well-bred corporate types.

Still think Alberg made the all time most beautiful, natural boat body shape of any designer. For the rest of us. Even if he had had a computer and his 'flow solver was based on finite volumre discritization with a block structured mesh and a multi-stage explicit time integration using central differencing and multigrid acceleration'...

he still would have come up with the Ariel. Just wondering if he put a barn door on it.

[Brendan Watson]

Hi, David,
I rebuilt my Commander rudder and made it larger also.
the feel of the deteriorated original felt vague. I went with a
rounded profile with the main addition beneath the rudder-strap.
Weather-helm is not reduced in terms of feel, its increased. Helm
power is well increased as are the loads on the rudder etc., and the
helmsman. The extreme is when backing under power. When the
rudder is turned from one side, over center and then to the other the
rudder un-loads at center then, with any stern-way, loads up big time.
If you dont feather and brake the tiller the rudder would just slam over.
Bigger works for me and if you can get over to City Island in the next two weeks
You can see mine.
As for the rudders shape I built a laminated plywood epoxy rudder.
For strength and thickness the shaft side is four layers of 3/8" mahoghany
ply. The two outer plys end vertically about a foot aft of the shaft. In this thick,
solid area go the bolt holes. Aft of this, the outer plys are balsa -core, nice and easy to shape.
I'm sure a more efficient rudder foil exists, I tried to shape mine to be in harmony
with the rest of the boats design which I think is very natural and contributes to
the kind manners of our boats. Good luck!

Cheers, B.
Commander 215

[Bogle]

Brendan,

Thanks for the input. I am not able to come see your rudder, Brendan, but thanks for the offer. I barely have the time to finish building mine. I do hope to see you at the Sand Hole this season, however.

I've just epoxied the three planks together. Next step is cutting the shape, drilling for the drift pins, planning the taper, then bolt and pin it all together. I will post more pages eventually on the steps of construction.

The shape will follow the original rudder's upper half, and add some area at the lower edges, per my posted pattern. The original shape and area will be able to be recreated by cutting it down slightly. I'll give it a season at least before trying that.

[walberts]

Did you already try reducing weather helm by adjusting the rig? If you have weather helm (meaning the boat's tendency to turn up into the wind) the mast is probably raked aft a bit. Bring it into perpendicular and she should balance out. If it is still a problem, try raking the mast very slightly forward.

The weather helm is caused by the Center of Effort of the sail plan being behind the Center of Lateral Resistance of the keel. So the stronger the wind, the more the boat wants to head up. If the mast is perpendicular, the boat should balance. If she doesn't, try raking the mast forward which will bring the CE and the CLR into alignment and the boat will not fight you when the wind pipes upl But be careful. If the mast is too far forward, the boat will develop a lee helm, which is equally undesirable as a weather helm if not more so, because it is potentially more dangerous.

A little weather helm is not a bad thing.

[Bill]

As has been noted, Ebb mocked up a new rudder for 338 using the one illustrated in Alberg's original drawing. [see post 83 & 84 at http://www.pearsonariel.org/discussi...ht=new+rudder]

The May-June '06 edition of Good Old Boat mag has an article by Hal Roth (pg 25) where he describes converting the rudder on his Spencer 35 from a long curve to a straight line as shown in Ebb's mockup. Roth quotes navel architect German Frers, Sr. as advising him to "make the (new) rudder with straight lines like a 12-Meter." In essence, the navel architects are saying that our original rudder shape is obsolete and that a straight rudder design will better steer the boat because it has more area down low. Probably less drag, too.

[drm901]

My Ariel tracks fantastic, no weather helm. In fact, that is what I like about the design so much. My former boat was a challenge to steer when the winds picked up. The rudder is original, so I think your issue is the tuning of the rig, not the rudder.